In the automotive industry, open, welded profiles are increasingly being replaced by thin-walled hollow profiles, the starting shapes of which consist of longitudinally welded tubes. Owing to small wall thicknesses, these components are designed such that a minimum weight is achieved with the highest exploitation of the material.
So that the technical functionality of so-called space/frame structures can be ensured, the production process of the components must be controllable through to the final shaping.
An important element of the process chain is, in this case the profile forming process, in which the tube elements being used as hollow profiles are formed. The discontinuous mode of operation, in other words forming from ready-cut blanks has proven successful for forming special profiles, so-called “tailored tubes”. Tailored tubes are tube elements of the type which are composed of sheet metal portions, or their metal properties are adapted to the loads and requirements occurring during practical use or in the shaping process.
To form sheet metal blanks into finished welded profiles, there are various possibilities. Most solutions, however, use separate working stations for the forming and the welding (DE 44 32 674 C1).
A device of the type mentioned above, which allows the production of longitudinally welded tubular elements in a station is known from DE-PS 966 111, for example. In this device, the sheet metal blank is shaped to form an open seam tube and welded. For this purpose, a flat metal sheet is held between two tool halves which can be moved with respect to one another on a tool carrier, are arranged mirror symmetrically with respect to one another and in each case have a half shell-shaped recess determining the outer contour of the tube to be produced. The longitudinal edges of the sheet metal blank are, in this case, aligned parallel to the recesses of the tool halves in such a way that the tool halves receive the edges associated with them in each case when moved together and, viewed in cross-section, move them toward one another in the manner of an arc of a circle. During the moving together, the sheet metal blank is fixed by holders, which are positioned at the ends of the centre longitudinal axis of the sheet metal blank, which are associated with the two short edges of the sheet metal blank. It is thus ensured that the sheet metal blank, as a result of the moving together of the tool halves, in a uniform movement shifts along the contour predetermined by the recesses of the tool halves until its longitudinal edges meet one another at the apex point. The open seam tube thus formed is held in this position to weld its longitudinal edges. To make the open seam region of the open seam tube accessible, the upper end portions of the tool halves are folded up. The longitudinal edges associated with one another in the open seam region are then longitudinally welded to one another.
A decisive disadvantage of the procedure known from DE-PS 966 111 is that it assumes a certain minimum stiffness of the processed metal sheet. Only in this way is it ensured that, when the tool halves are moved together, the metal sheet curves uniformly to form the open seam tube. Thin metal sheets cannot be shaped in this known manner in a targeted manner, to form a tube with a accurately predetermined cross-sectional shape, but during the course of the shaping process form uncontrollable edges and folds, which may render the tube element obtained unusable. The welding of an open seam tube formed according to the method known from DE-PS 966 111 also leads to undesired shapings when the metal sheet is so thin that it cannot absorb forces unavoidably acting on it during welding.
An attempt to eliminate the disadvantages of devices of the type described above, is known from WO 99/67037. The procedure known from this document connects the forming and welding in one working station. For this purpose, a tool is used, which according to the model of DE-PS 966 111, has two tool halves which can be moved with respect to one another on a tool carrier, with a cylindrical half shell-shaped recess in each case. In addition, an inner mandrel half is associated in each case with the recesses of the tool halves, which inner mandrel half is positioned in the respective recess leaving free a gap between its outer face and the inner face of the recess and rigidly connected to the respective tool half. An annular gap is formed in this manner in the region of the recess of each tool half.
To form the tube element, the thin metal sheet to be shaped is placed between the tool halves in such a way that, during the subsequent moving together of the tool halves, its longitudinal edges are threaded into the annular gap formed in the respective tool half. When the tool halves are moved together further, the longitudinal edges shift further up the longitudinal gap and the metal sheet is bent to form an open seam tube. The support of the sheet metal blank taking place simultaneously on the inner and outer side of the sheet metal portion, in the region of the annular gap, ensures here that no undesired fold and edge formation occurs.
This forming process called “rolling-in” is completed when the longitudinal edges meet in the apex of the open seam tube obtained. Following after-rounding of the edge joint by means of a roller, this open seam tube can be welded in the region of its open seam, without the tube having to be brought into another device for this purpose.
The method known from WO 99/67037 is supposed to allow the production of accurately shaped tube elements, but has some disadvantages in practice, however. Thus, the divided design of the inner mandrel and the configuration of the annular gap in the region of the recesses of the tool halves requires high production accuracy in the production of the tools. The configuration of the machine is substantially more complex as to separate the finished welded tube from the divided inner mandrel expensive pulling units are required and special, very powerful holding-down devices are required for holding the mandrel position. The relative movement between the tool and metal sheet in the region of the unavoidably narrow annular gap also leads to damage to the surface of the metal sheet and to wear of the tools. Moreover, in particular when processing thin metal sheets, there is still the risk that during welding undesired shapings may occur in the region of the weld seam during welding because of the edge stresses occurring in the process.